BasicStudents matched to this level have a partial mastery of prerequisite knowledge and skills fundamental for proficient work.

At Grade (Proficient)Students matched to this level have demonstrated competency over challenging subject matter, including subject matter knowledge, application of such knowledge to real-world situations, and analytical skills appropriate to subject matter.

AdvancedStudents matched to this level are ready for material that requires superior performance and mastery.

Developing plate tectonics theory took several decades and the work of many scientists. Continental drift was formulated by Alfred Wegener and seafloor spreading was developed by many scientists, including Harry Hess.

When North and South America joined at Panama, animals that had been separated for millions of years were in contact. North American mammals, which gave live birth, outcompeted the South American marsupials.